1. Field of the Invention
The present invention relates to improved methods of fracturing high temperature subterranean zones and viscous aqueous foamed fracturing fluids therefor.
2. Description of the Prior Art
A treatment commonly utilized for stimulating hydrocarbon production from a subterranean zone penetrated by a well bore is hydraulic fracturing. In a hydraulic fracturing treatment, a viscous treating fluid, referred to in the art as a fracturing fluid, is pumped through the well bore into the subterranean zone to be stimulated at a rate and pressure such that fractures are formed and extended in the subterranean zone. A portion of the fracturing fluid has proppant particles suspended therein which are carried into and deposited in the formed fractures when the viscous fracturing fluid is broken and recovered. The proppant particles function to prevent the foamed fractures from closing whereby conductive channels are formed through which produced fluids can flow to the well bore. The breaking of the fracturing fluid, i.e., the reduction in the viscosity of the fracturing fluid whereby it can be recovered, is accomplished by adding a delayed breaker to the fracturing fluid prior to pumping it into the subterranean zone. The delayed breaker effects a controlled reduction in the viscosity of the fracturing fluid so that the proppant particles therein are deposited in the fractures and the fracturing fluid is recovered.
Viscous aqueous foamed fracturing fluids have been utilized heretofore. The benefits of using foamed fracturing fluids instead of non-foamed fracturing fluids are many including reduced leak off of the fracturing fluid into permeable formations, less damage to the subterranean zone being fractured as a result of polymer residue entering the zone permeability and/or the proppant particle packs formed in the fractures, and lower density of the fracturing fluid which facilitates the flow back of the fluid after its viscosity has been reduced.
The gases utilized in forming foamed fracturing fluids have primarily been nitrogen, carbon dioxide and mixtures thereof. Carbon dioxide becomes more economical to use in wells having greater depths and correspondingly higher temperatures and pressures. When nitrogen is utilized in a foamed fracturing fluid at high pressures, greater quantities of nitrogen must be compressed to maintain a high quality foam. Carbon dioxide, on the other hand, is pumped as a liquid or a very dense gas so that it does not require as much compression in high pressure wells as nitrogen. In addition, the density of carbon dioxide allows it to be pumped at lower well head pressures than nitrogen.
Carbon dioxide foamed fracturing fluids have heretofore been utilized in subterranean zones having temperatures up to about 400° F. However, the viscosity of a foamed fracturing fluid is dependent upon the liquid phase thereof and the viscous liquids utilized heretofore have generally been unstable at temperatures above about 300° F. Aqueous gelled liquids containing gelling agents such as guar, hydroxypropylguar and carboxymethylhydroxypropylguar lose viscosity by thermal thinning and become hydrolytically unstable above about 350° F. Also, at 350° F. and above, the heretofore used carbon dioxide foamed fluids have not had good proppant particle carrying capability.
Thus, there are continuing needs for improved carbon dioxide foamed fracturing fluids that maintain their viscosities at temperatures up to 400° F. and higher and have good proppant particle carrying ability.